1. Field of the Invention
The present invention relates to a continuously variable transmission, and more particularly, it relates to a V belt continuously variable transmission for an automobile.
2. Description of the Related Art
Generally, a continuously variable transmission has been proposed, as described in Japanese Laid Open Patent No. 64-58848 (1989) for example, which includes a driving pulley, a driven pulley, a metallic belt placed around the driving and driven pulleys, a pressure regulating mechanism attached to at least one of those pulleys for exerting an axial force corresponding to the applied torque, and stroke mechanisms (actuator mechanisms) attached to the driving and driven pulleys respectively for moving the movable sheaves in the axial direction, and a shifting device for operating the stroke mechanisms at the time of changing the transmission ratio.
This shifting device includes a motor, an operating shaft having a pair of gears connected to the stroke mechanisms of the driving and driven pulleys respectively, a reduction mechanism consisting of reversible gear train which connects the motor to the operating shaft, and an electromagnetic brake which keeps an output shaft of the motor suspended when the motor is not in operation.
However, because the metallic belt used in this transmission has considerably small coefficient of friction μ against the pulleys, the pressure regulating mechanism, such as a pressure regulating cam mechanism, should necessarily be attached to at least one of the pulleys for transmitting the torque. The pressure regulating mechanism presses the fixed sheave from behind so that the belt comes into pressure contact with the pulley. Nevertheless, this pressing force, being transmitted to the stroke mechanism via the belt and the movable sheave, exerts heavy loads on the stroke mechanism. Screw means such as a ball screw mechanism is used as the stroke mechanism. However, when such screw means is operated under a large axial force, the screw means is worn away fast, resulting in reduced durability. Further, when such a large axial force is exerted on the stroke mechanism, a larger rotational force is required for operating the stroke mechanism. Consequently, the motor should be larger in its size and in its consumption of electric power. Even when the transmission efficiency of the reduction mechanism is improved, it does not always lead to miniaturization or power saving of the motor.
Besides the above-described metallic belt (wet-type belt), a dry-type belt, which has the friction surface consisting of resin or rubber, etc., for touching with the pulleys, can also be employed. Because the metallic belt is lubricated with oil, the coefficient of friction μ between the belt and the pulleys is small, whereby strong clamping forces generated by the pressure regulating mechanism should be exerted on the metallic belt in order to achieve the prescribed transmission efficiency. On the contrary, the dry-type belt which is not lubricated with oil has a large coefficient of friction μ against the pulleys, whereby a high level of transmission efficiency can be achieved without the strong clamping forces on the belt by the pulleys.
Accordingly, an object of the present invention is to provide a continuously variable transmission using a dry-type belt that is capable of solving or preventing the aforementioned problems.